The present invention relates to a process for phospholipidation of imidazoquinolines and oxoadenines. More particularly, the present invention relates to a high-yielding and scalable procedure for the phospholipidation of imidazoquinolines and oxoadenines which obviates the need to isolate unstable phosphoramidite intermediates. This process may be used for the phospholipidation of toll-like receptor 7 (TLR7)—active and toll-like receptor 8 (TLR8)—active imidazoquinolines and oxoadenines.
Toll-like receptors (TLRs) are a family of more than 10 structurally related receptors on innate immune cells that detect pathogen-specific components common to large classes of microbial invaders. Activation of these receptors leads to the expression of inflammatory cytokines/chemokines and type I interferons alpha and beta (IFNα/β) important for effective innate and adaptive immune responses to infectious disease and cancer.
In the case of TLR7 and TLR8 activation, a few different classes of small-molecule mimetics of the natural uridine- and/or guanosine-rich viral ssRNA ligands have been identified (Heil et al. Eur. J. Immunol. 2003, 33, 2987-2997, Hemmi et al. Nat. Immunol. 2002, 3, 196-200, Lee et al. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 1828-1833), including oxoadenine analogs (Hirota et al. J. Med. Chem. 2002, 45, 5419; WO 2007/034882 PCT/JP2006/318758, Dainippon Sumitomo Pharma Co. Ltd./AstraZeneca Aktiebolag) and 1H-imidazo[4,5-c]quinolines (Gerster et al. J. Med. Chem. 2005, 48, 3481-3491) such as imiquimod, which is approved for topical treatment of certain skin diseases and known to primarily activate TLR7, and the structurally related imidazoquinoline resiquimod, which potently activates both TLR7 and TLR8 (Miller et al. Drug News Perspect. 2008, 21, 69-87).
Since TLR7 and TLR8 are broadly expressed in dendritic cells (DCs) and other antigen presenting cells, TLR7/8 agonists and their derivatives may be especially useful as vaccine adjuvants. However, oral and topical preparations of imiquimod and resiquimod and other small-molecule TLR7/8 agonists can exhibit serious side effects, and clinical trials with certain TLR7/8 agonists have been suspended over safety concerns (Horscroft et al. J. Antimicrob. Chemother. 2012, 67, 789-801, Strominger, N. L.; Brady, R.; Gullikson, G.; Carpenter, D. O. Brain Res. Bull. 2001, 55, 445-451). In addition, since TLR7 and TLR8 are located in endosomal/lysosomal compartments (Lee et al. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 6646-6651), cellular uptake is prerequisite for cellular activation by TLR7/8 ligands. Thus, there is considerable interest in strategies that will increase the penetration of the TLR7/8 ligand into DCs and other immune cells as well as ameliorate toxic effects. Lipid conjugation of nucleoside drugs including TLR7/8 agonists (Chan et al. Bioconjugate Chem. 2009, 20, 1194-1200) is one strategy known to facilitate endocytosis, enhance oral bioavailability, and decrease toxic side effects. Such nucleolipids can also be incorporated into liposomes and other biodegradable nanoparticles to help protect the drug from degradation and further reduce toxicity through a depot effect (Rosemeyer, H. Chemistry & Biodiversity 2005, 2, 977-1063).